Page 1 of IRS Instructions for Form 8839: "An eligible child is: Any child under age 18. If the child turned 18 during the year, the child is an eligible child for the part of the year he or she was under age 18."

U.S. Master Tax: "Eligible Child and Special Needs Child An eligible child for the adoption credit is an individual who has not attained the age of 18 as of the time of the adoption, or who is physically or mentally incapable of caring for himself or herself."

Quickfinder: “If the child turns 18 before the adoption is finalized, only expenses incurred before the child turned 18 will be eligible for the credit. 

In this particular case, all of the adoption expenses were incurred after the child's 18th birthday.  No COVID delay.